The present invention generally relates to gas turbine engine combustor systems and, more particularly, to combustor heat shields.
Combustor heat shields have been used extensively in gas turbine engines. These heat shields are positioned between the combustor dome and the combustion chamber and are used to shield the combustor dome from the heat generated by combustion gases within the combustion chamber. There is a cooling gap between the heat shield and the combustor dome. Impingement openings in the combustor dome allow air to pass through the dome and into the cooling gap. This air then contacts the upstream surface of the heat shield, cooling the heat shield by impingement and convection heat transfer. High hoop stresses usually result from steep thermal gradients in both the radial and the xe2x80x9cthru-the-wallxe2x80x9d directions of the heat shield, particularly during transient thermal cycling occurring during engine power changes. Cyclic exposure to combustion gases causes heat shield creep and distortion. Heat shield creep and distortion is accelerated by cooling gap closure. As the heat shield distorts, the cooling gap begins metering flow that results in reduced and non-uniform cooling of the heat shield and combustor dome. The heat shield then collapses onto the dome and heat shield replacement is necessary. If the heat shield is not replaced, there is total loss of cooling to the combustor dome. Elevated heat shield temperatures accelerates oxidation of the heat shield and combustor dome resulting in reduced resistance to cracking.
Some heat shields are not easily accessible and the entire dome system must be replaced. Because replacement of the entire dome system is costly and frequent heat shield replacement is necessary, easily disassembled dome systems have been described. Although, this reduces the cost associated with replacing the heat shield, it does not reduce the need for frequent heat shield replacement.
Increasing the durability of the heat shield will reduce replacement frequency. Adding stiffeners to the heat shield is one known method of reducing thermal distortion of the heat shield. Other methods of increasing heat shield durability include increasing the volume of air that passes through the impingement openings. The increased air volume will increase heat shield cooling, but the quantity of cooling air available to the heat shield is generally restricted by the demand to cool other areas of the combustor and turbine. Additional methods of increasing heat shield durability include using stronger materials to form the heat shield and the incorporation of more effective cooling techniques.
Heat shield durability has been improved by increasing heat shield cooling effectiveness. Effusion holes have been placed in the heat shield, such that air passes from the cooling gap to the downstream surface of the heat shield there through. The air then forms a cooling air film on the downstream surface of the heat shield. Although this increases heat shield cooling effectiveness, the quantity of cooling air required may be prohibitive or all areas of the heat shield may not be sufficiently cooled. Also, the effusion holes weaken the structure of the heat shield.
A heat shield for a gas turbine combustion chamber has been disclosed in U.S. Pat. No. 5,956,955. The described system includes effusion holes that are positioned at an incline and a ring-shaped channel between the heat shield and the burner. Swirling air exits the cooling gap into the ring-shaped channel and then into the combustion chamber. This is said to intensively cool the particularly highly stressed areas of the heat shield. Although this system is said to cool the through-hole edge area of the heat shield, improved heat shield cooling is still needed. The through-hole edge area, or inner diameter area, is the area of the heat shield that surrounds the fuel nozzle. The outer diameter area of the heat shield is also highly stressed and is not sufficiently cooled by this system.
Another heat shield has been described in U.S. Pat. No. 5,509,270. The wedge shaped heat shield disclosed includes slots extending radially out from the fuel nozzle through-hole edge. Although these slots alleviate excessive compressive hoop stress on the inner diameter of the heat shield, this system does not sufficiently address heat shields that experience high hoop stresses at the outer diameter where edge distortion can be severe.
Annular heat shields with ribs positioned on the outer diameter area have been disclosed in U.S. Pat. No. 6,032,457. The ends of the ribs are accurate in shape and are said to reduce the film cooling air vortices. Although these heat shields may provide an increased rate of heat transfer by convection, still further improvements are needed. Additionally, the described heat shields do not sufficiently reduce the mitigating effects of cross-flow, in a cooling scheme design where there is impingement cooling at the outer edge of the heat shield. Cross-flow is the convergence of the air flows coming from different impingement openings. Cross-flow creates disturbance that reduces heat shield cooling efficiency. Unfortunately, these heat shields do not sufficiently reduce the disturbance caused by cross-flow.
As can be seen, there is a need for improved heat shield durability. Also, heat shields are needed wherein the mitigating effects of cross-flow are reduced. Moreover, there is a need for increased heat shield cooling without an attendant increase in the quantity of allocated cooling air or compromising ease of manufacture. Further, an inexpensive heat shield having improved cooling of the outer diameter area is needed. Heat shields having reduced hoop stresses and reduced temperature gradients are also needed.
In one aspect of the present invention, a cooling apparatus for a combustor dome comprises: an annular heat shield; and a plurality of baffles positioned on an upstream surface of the annular heat shield, the baffles are U-shaped and each baffle comprises an arc and two ribs, the arc is radially inward from the two ribs, the combustor dome has an outer row and at least one inner row of impingement openings there through.
In another aspect of the present invention, an apparatus for a turbine engine comprises: an annular heat shield; a plurality of baffles on an outer diameter area of the annular heat shield; and a plurality of slots extending radially inward from an outer rim of the annular heat shield, each slot having a keyhole positioned at a radially inward end of the slot.
In still another aspect of the present invention, a shielding system for a combustor dome comprises: an annular heat shield; and at least one U-shaped baffle positioned on an upstream surface of the annular heat shield, the U-shaped baffle is clocked such that the U-shaped baffle is capable of segregating an air flow, the U-shaped baffle comprises an arc and two ribs, the arc is radially inward from the two ribs, the combustor dome has an outer row and at least one inner row of impingement openings there through.
In yet another aspect of the present invention, a cooling apparatus for a combustor dome comprises: an annular heat shield formed from a metal selected from the group consisting of (SC)180, HA230, Mar-M-247 Eqx, and MA754; a plurality of baffles positioned on an upstream surface of the annular heat shield, the baffles each comprise an arc and two ribs, the ribs have a width between about 0.020 inches and about 0.030 inches, the combustor dome has an outer row and at least one inner row of impingement openings there through, the baffles are clocked such that the baffles are capable of segregating an air flow from the outer row of impingement openings from an air flow from an inner row of impingement openings; a plurality of slots extending radially inward from an outer rim of the annular heat shield; and a plurality of keyholes through the annular heat shield, such that there is one keyhole at an inner end of each slot.
In a further aspect of the present invention, a method of cooling a combustor dome comprises the steps of: providing an annular heat shield downstream of the combustor dome, the combustor dome has an inner and an outer row of impingement openings there through, the inner row is capable of producing an inner row air flow, the outer row is capable of producing an outer row air flow; and positioning a plurality of baffles on an upstream surface of the annular heat shield, such that the inner row air flow is segregated from the outer row air flow.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.